parent and has a relatively simple organismal structure, at least for
an animal that is able to manifest some degree of behavioral sophis-
tication. Behaviors of C. elegans include navigation through the soil
environment in which they live using olfactory and thermal cues.
The nervous system of C. elegans is pretty simple; there are only 302
neurons, and the location, connectivity, and developmental history
of each and every one of them have been determined by researchers.
That is, the complete wiring diagram of the C. elegans nervous system
is known.
A bit larger (typically several millimeters long), and quite a bit more
complex in structure than nematodes, are the planaria, flatworms
found in both aquatic and terrestrial environments. The planarian
nervous system contains an extended network of interconnected
neurons, together with two clusters of neurons at the head end of the
worm (Fig. 2.3). Some neurobiologists consider these clusters to repre-
sent a primitive brain.
Insects clearly have complex brains. They also execute complex
behaviors. The tiny fruit fly Drosophila has been extensively studied
by biologists for more than a century and has been a focus of neurobi-
ological research for several decades now. Its brain is less than 0.5 mm
in width and contains around 150,000 neurons.
Many hundreds of millions of years of evolutionary variation and
selection have formed the backstory for the brains of contemporary
vertebrate animals—fish, amphibians, reptiles, birds, and mammals.
The basic structure of all vertebrate animal brains is similar and can
be represented as shown in Figure 2.4. The vertebrate brain develops
in the embryo when a tubular structure (the neural tube) folds in and
then closes off and expands at one end (on the left side in the diagram
in Fig. 2.4). The interior spaces of the tube will become the ventricles